Where to begin...there is so much wrong with this—and so much right!
First off, we see the problem with the allegorical interpretation of scripture. Mary and Martha are reduced to types. But, if Mary represents the future, then how come she's experiencing it right now?! She is already experiencing a restful life in Christ, sure there will be more in the future, as both Augustine and Cassian acknowledge. But, there is indeed a foretaste of it now.
Second, Augustine is wrong to say that it is rare and barely begun! This flows out of his extreme embrace of the fallenness of humanity. His earlier stuff, before he encountered Pelagius, is a bit more optimistic. But once he began to refute Pelagius, he became more extreme on the extent of the fall. I'm not enough of an Augustine scholar to know if he ever says that the image of God is destroyed in humanity, but he certainly extends the extent to which is it effaced. Cassian is closer to right here. We can experience a much greater degree of the contemplative life here and now than Augustine allows here.
What's right? We only get a foretaste. But what a foretaste it can be! But it's still only a foretaste.
Now, I wonder how Augustine reconciles his extreme pessimism here with his optimism here?!